It is well known that the productivity of an oil or gas well may often be increased by a procedure which involves creating a fracture in the subterranean formations surrounding a well and propping the fracture opening by filling it with granular material called propping agent. Methods of this type are disclosed in U.S. Pat. Nos. 3,701,383 and 4,068,718.
A survey of propping agents and their manufacture is given in the specification to U.S. patent application Ser. No. 914,647 filed the Dec. 6, 1978 now abandoned.
A granular material must fulfill several conditions to be suitable for use as a propping agent. The material must have high strength to avoid crushing of the particles when exposed to high pressure during their application. The shape of the individual particle should depart as little as possible from the spherical one and the particle size distribution should be within defined, relative narrow limits to insure sufficient gas and oil permeability of fractures propped with the propping agent. Moreover, the particles should be able to resist the corrosive conditions to which they may be exposed at their application.
The material regarded as most suitable for fulfilling these conditions is sintered bauxite pellets.
Several methods have been proposed for producing sintered bauxite particles. The process which has hitherto found widest commercial success is the one described in the above U.S. application Ser. No. 914,647.
According to said methods bauxite spheres are first prepared by agglomeration of a mixture of bauxite, temporary binder and water in an intensive mixer to produce spheres called green pellets, which are afterwards sintered by heating. In the embodiment examples of said application the products have a typical density of 3.7 g/cm.sup.3, while the crushing loss measured by the method described below was 8.16% and 6.8% respectively.
It is however, a drawback of said process that the granulation, which is carried out in an intensive mixer, can only be performed batchwise. Moreover, said prior art process usually requires a preliminary drying of the starting material.